Conversion of direct process high boiling component to silane monomers in the presence of hydrogen gas

Abstract: A process for converting a high-boiling component resulting from the reaction of an organochloride with silicon into commercially desirable monosilanes is described. The process comprises contacting the high-boiling component with hydrogen gas at a temperature within a range of 250°C. to 1000°C. Process yield may be improved by use of a catalyst selected from activated carbon, platinum metal, platinum supported on alumina, palladium supported on carbon, SbCl₅, H₂PtCl₆, BCl₃, AlCl₃ and AlCl₃ supported on a support material selected from carbon, alumina and silica. A preferred embodiment of the process is run at a gauge pressure of 1.7 to 6.9 MPa (250 - 1000 psi). This process is preferential for the production of diorganodichlorosilane in relation to organotrichlorosilane in the monosilane product.

Description: The present invention is a process for converting a high-boiling component, resulting from the reaction of an organochloride with silicon, into more commercially desirable monosilanes. The process comprises contacting the high-boiling component with hydrogen gas at a gauge pressure of 1.7 - 6.9 MPa (250 - 1000 psi) at a temperature within a range of 250°C. to 1000°C. Process yield is improved by use of a catalyst selected from activated carbon, SbCl 5 , BCl 3 , AlCl 3 and AlCl 3 supported on a support material selected from carbon, alumina and silica. A significant advantage of the present process is that it is preferential for the production of diorganodichlorosilane in relation to organotrichlorosilane in the monosilane product. The high-boiling component of the present process results from a process typically referred to as the "Direct Process," where an organohalide is reacted with silicon in the presence of a suitable catalyst to form monosilanes. The Direct Process is described more completely in US-A 2,380,995 and US-A 2,488,487. It is the main commercial process by which organohalosilanes (i.e. monosilanes), for example dimethyldichlorosilane and trimethylchlorosilane are formed. These organohalosilanes are reactive compounds which can undergo numerous reactions to form a variety of useful silicon containing compounds and polymers. In typical commercial direct processes the process is optimized to produce the diorganodihalosilane, since this particular monosilane can be hydrolyzed to form polysiloxane polymers having a broad range of commercial applications. Polysiloxane polymers are useful as heat transfer fluids, lubricants and can be further processed to form silicone elastomers, resins, sealants and adhesives. Operation of the Direct Process results not only in the production of the desirable monosilanes, but also in a high boiling component typically considered to be all materials with a boiling point higher than the particular diorganodihalosilane produced in the process. The high-boiling component is a complex mixture that includes compounds containing SiSi, SiOSi, SiCSi, SiCCSi and SiCCCSi linkages in the molecules. Typical compounds found in a high-boiling component are described in US-A 2,598,435 and US-A 2,681,355. The high-boiling component may also comprise silicon containing solids and soluble and insoluble compounds of copper, aluminum and zinc. In current commercial operations for performing the Direct Process, the high-boiling component can constitute as much as ten percent of the resultant product. Therefore, it is desirable to convert the high-boiling component into more commercially desirable products to both reduce low value by-products and to improve raw materials utilization. The prior art for converting the high-boiling component of the Direct Process into more usable monosilanes is represented by US-A 2,598,435; US-A 2,681,355; US-A 2,709,176; US-A 2,842,580; US-A 3,639,105; US-A 4,059,608 and US-A 4,079,071. FR-A-1 590 075 discloses a process for preparing monosilanes by thermal degradation of residues resulting from the preparation of alkyl chlorosilanes, by using hydrogen and/or alkyl chloride at ambient pressure in a temperature range of from 300 to 600 °C in the presence of metal which acts as catalyst. Takeda et al., Kogyo Kagaku Zasshi (Journal of Industrial Chemistry), Vol. 60, No. 11, p. 1392-1395 (1957), also describe the use of alumina, carbon and pumice as catalyst for the catalytic cracking of disilane in a hydrogen stream. An objective of the present invention is to provide a process for the conversion of a high-boiling component from the direct process to useful monosilanes. A second objective is to provide such a process which is preferential for the production of diorganodichlorosilane in relation to organotrichlorosilane in the monosilane product. The present invention comprises contacting a high-boiling component, resulting from the reaction of an organochloride with silicon and hydrogen gas at a gauge pressure within a range of 1.7 to 6.9 MPa (250 to 1000 psi) at a temperature within a range of 250°C. to 1000°C. Yield of the present process is improved by use of a catalyst selected from a activated carbon, SbCl 5 , BCl 3 , AlCl 3 and AlCl 3 supported on a support material selected from carbon, alumina and silica. The present process may be run in any standard type reactor for contacting chlorosilanes with a gas such as hydrogen. The process may be run as a batch process, semi-continuous or continuous process. The process can be run, for example, in a fixed-bed reactor, a stirred-bed reactor or a fluidized-bed reactor. The present process is useful for converting a high-boiling component resulting from the reaction of an organochloride with silicon to monosilanes. The term "high-boiling component" refers to those materials with a boiling point above that of the diorganodichlorosilane formed by the reaction of the organochloride with silicon. For example, when methyl chloride is reacted with silicon, the diorganodichlorosilane will be dimethyldichlorosilane and the high-boiling component will comprise those materials having a boiling point greater than that of dimethyldichlorosilane, i.e. greater than 70°C. In a typical process for reacting an organochloride with silicon, the reaction is conducted at a temperature of 270°C. to 350°C., in the presence of a suitable catalyst. Gaseous products and unreacted feed are continuously removed from the process. The removed gaseous products and unreacted feed are subsequently distilled to remove monosilanes, leaving behind a high-boiling component. The high-boiling component is a complex mixture that includes compounds containing SiSi, SiOSi, SiCSi, SiCCSi and SiCCCSi linkages alone or in combination in each molecule. The high-boiling component can include silicon containing solids and soluble and insoluble compounds of copper, aluminum and zinc. The high-boiling component may contain, for example, organic substituted and non-organic substituted silanes, disilanes, trisilanes, disiloxanes, silane oligomers, siloxane oligomers, silalkylenes and silicon containing solids, all of which may be converted to monosilanes by our process. The present process is especially useful for converting polysilanes in the high-boiling component to monosilanes, where the polysilanes are described by formula R a H b Si n Cl 2n+2-a-b and where each R is a radical independently selected from alkyls comprising one to six carbon atoms, n=2 to 20, a=0 to 2n+2, b=0 to 2n+2 and a+b=0 to 2n+2. The polysilanes useful in our process can consist of n number of silicon atoms where n is an integer from two to 20. Preferred is when n equals two. The polysilanes can be substituted with a=0 to 2n+2 number of R radicals, where each R is independently selected from alkyls of one to six carbon atoms. The radical R can be methyl, ethyl, propyl and t-butyl. Preferred is when R is methyl. The polysilanes in the high-boiling component can contain b number of hydrogen atoms substituted on the silicon atoms, where b=0 to 2n+2. The polysilanes in the high-boiling component can also contain from zero to 2n+2 chlorine atoms. The high-boiling component can contain silalkylenes, where each silalkylene can comprise one or more silakylene bonds described by formula Si(C) z Si and z is an integer from one to six. Preferred is when z is an integer from one to three. The silicon atoms of the silalkylene molecules can be further substituted with the radical R, where R is as previously described, with chloride and with hydrogen. Preferred is when the silicon atoms of the silalkylenes are substituted with methyl. The preferred high-boiling component is one resulting from the reaction of methyl chloride with silicon, the high-boiling component having a boiling point greater than 70°C. This high-boiling component can contain Me 2 ClSiSiMe 2 Cl, Me 2 ClSiSiMeCl 2 , MeCl 2 SiSiMeCl 2 , Me 2 ClSiSi(Me)(Cl)SiMeCl 2 , Me 2 ClSiCH 2 SiMe 2 Cl, Me 2 ClSiCH 2 SiMeCl 2 , MeCl 2 SiCH 2 SiMeCl 2 , Me 2 ClSi(CH 2 ) 2 SiMeCl 2 , Me 2 ClSi(CH 2 ) 3 SiMeCl 2 , Me 2 ClSiCH 2 Si(Me)(Cl)SiMeCl 2 , Me 2 ClSiCH 2 Si(Me)(Cl)CH 2 SiMeCl 2 and Me 2 ClSiOSiMeCl 2 , where Me is methyl, all of which may be converted to monosilanes by the present process. The high-boiling component is contacted with hydrogen gas, where the ratio of the weight of high-boiling component added to the reactor to the weight of hydrogen gas is within a range of 0.1:1 to 1000:1. Preferred is where the ratio of the weight of high-boiling component to the weight of hydrogen gas is within a range of 1:1 to 500:1. The present process can be run at a gauge pressure within a range of 1.7 to 6.9 MPa (250 to 1000 psi). Generally, pressures within the above range favor the production of the diorganodichlorosilane over the organotrichlorosilane. The process is conducted at a temperature within a range of 250°C. to 1000°C. A preferred temperature is within a range of 270°C. to 650°C. The process is conducted in the presence of a catalyst selected from activated carbon, SbCl 5 , BCl 3 , AlCl 3 and AlCl 3 supported on a support material selected from carbon, alumina and silica. The weight of catalyst in relation to the weight of high-boiling component and hydrogen gas added to the process will depend upon such factors as the type of catalyst, the chemical composition of the high-boiling component, the process temperature and the type of reactor employed. The optimum contact time for the high-boiling component and hydrogen gas with the catalyst will depend, for example, on factors such as the type of catalyst, chemical composition of the high-boiling component and the degree of conversion and product selectivity desired. In general, contact times within a range of one second to five hours are useful. Longer contact times may be employed, but appear to offer no advantage and may result in excessive scission of silicon-carbon bonds and silicon-hydrogen bonds present in the monosilanes. A preferred contact time in a continuous-reactor system is within a range of one second to two hours. If desired, the monosilane containing product of the present process can be further separated by standard means, for example, distillation to separate the monosilanes from a high-boiling component and the high-boiling component can then be recycled to the process. Monosilanes which can be produced by the present process are described by the formula R x H y SiCl 4-x-y , where R is as previously described (i.e. each R is independently selected from alkyls comprising one to six carbon atoms); x = 0, 1, 2 or 3; y = 0, 1 or 2 and x + y = 0, 1, 2, 3 or 4. The preferred monosilanes produced in the process are triorganochlorosilanes, diorganodichlorosilanes and organodichlorosilanes. Even more preferred is when the ratio of diorganodichlorosilane to organotrichlorosilane is 1.0 or greater in the monosilane product. The following examples are provided to facilitate understanding and to demonstrate the effectiveness of the present invention. Example 1 (Not within the scope of the present invention) A high-boiling component resulting from the reaction of methyl chloride with silicon was contacted with hydrogen chloride. The reactor consisted of a 2.5 cm diameter by 50 cm length quartz tube maintained at 500°C. The high-boiling component comprised by weight, 55% methylchlorodisilane, 5% disilmethylenes, 35% other polysilanes and silalkylenes and 5% silicon-containing solids. The high-boiling component was fed at a rate of 117 g/h to the reactor and hydrogen chloride was fed to the reactor at 50 g/h. The process was conducted for one hour with gaseous product exiting the reactor being collected in a cold condenser. The condensed product was analyzed by gas chromatography using a thermal conductivity detector (GC-TC) and found to consist of 59 weight percent chlorosilane monomers. Chlorosilane monomers detected included HSiCl 3 , SiCl 4 , MeHSiCl 2 , Me 3 SiCl, MeSiCl 3 and Me 2 SiCl 2 . The Me 2 SiCl 2 and MeSiCl 3 comprised 43 weight percent of the monosilane product. The ratio of Me 2 SiCl 2 to MeSiCl 3 was 0.6. Other species detected in the condensed product included methylchlorodisilanes, disilmethylenes, polysilanes and silalkylenes. Example 2 (not within the scope of the present invention) A high-boiling component resulting from the reaction of methyl chloride with silicon was contacted with hydrogen gas. The process was conducted similar to that described in Example 1, with the high-boiling component composition and reactor design being the same. The process was conducted at a temperature of 750°C. The high-boiling component was fed to the reactor at a rate of 91 g/h and hydrogen gas was fed to the reactor at a rate of 3 g/h. The process was conducted for one hour with gaseous product exiting the reactor being collected and analyzed as described in Example 1. The collected product was found to comprise 50 weight percent chlorosilane monomers and included MeSiCl 3 , Me 2 SiCl 2 , Me 2 HSiCl, HSiCl 3 , SiCl 4 , MeHSiCl 2 and Me 3 SiCl. The Me 2 SiCl 2 and MeSiCl 3 comprised 78 weight percent of the monosilane product. The ratio of Me 2 SiCl 2 to MeSiCl 3 was 1.3. Other species detected in the monosilane product included methylchlorodisilanes, disilmethylenes and other high-boiling silicon containing polymer species. Example 3 A high-boiling component resulting from the reaction of methyl chloride with silicon was contacted with hydrogen gas at a reactor gauge pressure of 6.9 MPa (1,000 psi) for 37 minutes. The reactor consisted of a 450 mL Hastaloy C™ pressure reactor. About 230 g of a high-boiling component, as described in Example 1, was added to the reactor along with 0.8 g of hydrogen gas. The reactor was maintained at 300°C. and approximately 6.9 MPa (1,000 psi) gauge pressure for 37 minutes. After the reaction period the reactor was vented to a condenser and the collected product analyzed as described in Example 1. The collected product was determined to consist of similar species as those described in Example 2. Of the product collected, 66 weight percent was chlorosilane monomers. The Me 2 SiCl 2 and MeSiCl 3 comprised 82 weight percent of the monosilane product. The ratio of Me 2 SiCl 2 to MeSiCl 3 was 2.7. Example 4 A high-boiling component resulting from the reaction of methyl chloride with silicon was contacted with hydrogen gas at a reactor gauge pressure of 6.9 MPa (1,000 psi) for two minutes. The reactor consisted of a 450 mL Hastaloy C™ pressure reactor. About 260 g of a high-boiling component, as described in Example 1, was added to the reactor along with 0.7 g of hydrogen gas. The reactor was maintained at 325°C. and approximately 6.9 MPa (1,000 psi) gauge pressure for two minutes. After the reaction period, the reactor was vented to a condenser and the collected product analyzed as described in Example 1. The collected product was determined to consist of similar species as those described in Example 2. Of the product collected, 50 weight percent was chlorosilane monomers. The Me 2 SiCl 2 and MeSiCl 3 comprised 85 weight percent of the monosilane product. The ratio of Me 2 SiCl 2 to MeSiCl 3 was 4.1. Example 5 (not within the scope of the present invention) A high-boiling component resulting from the reaction of methyl chloride with silicon was contacted with hydrogen gas in the presence of activated carbon catalyst. The process was conducted similar to that described in Example 1, with the high-boiling component composition and reactor design being the same. The reactor temperature was 500°C. The reactor was packed with 17 g of Caigon™ BPL lmm by 3.3 mm activated carbon pellets (Calgon, Pittsburgh, PA). The high-boiling component was fed to the reactor at a rate of 101 g/h and hydrogen was fed to the reactor at a rate of 3 g/h. The process was conducted for one hour with gaseous products exiting the reactor being collected and analyzed as described in Example 1. The collected product was determined to consist of similar species as those described in Example 2. Of the product collected, 36 weight percent was chlorosilane monomers. The Me 2 SiCl 2 and MeSiCl 3 comprised 91 weight percent of the monosilane product. The ratio of Me 2 SiCl 2 to MeSiCl 3 was 1.5.

Claims: A process for converting a high-boiling component resulting from the reaction of an organochloride with silicon, to monosilanes comprising diorganodichlorosilane and organotrichlorosilane, wherein the ratio of the diorganodichlorosilane and the organotrichlorosilane is 1.0 or greater, the process comprising contacting the high-boiling component with hydrogen gas at a gauge pressure within a range of 1.7 to 6.9 MPa (250 - 1000 psi) at a temperature range of 250 to 1000 °C, in the presence of a catalyst selected from activated carbon, AlCl 3 , SbCl 5 , BCl 3 , and AlCl 3 supported on a support material selected from carbon, alumina or silica, provided that there is not added any organotrihalosilane to said starting high-boiling component to be reacted with silicon. A process according to claim 1 where the ratio of the weight of the high-boiling component to the weight of the hydrogen gas is within a range of 0.1:1 to 1000:1. A process according to claim 1 where the high-boiling component results from the reaction of methyl chloride with silicon and the high-boiling component comprises polysilanes, silalkylenes, silicon-containing solids and soluble and insoluble compounds of copper, aluminum and zinc. A process according to claim 3 where the polysilanes are described by formula R a H b Si n Cl 2n+2-a-b , where each R is methyl, n = 2 to 20, a = 0 to 2n+2, b = 0 to 2n+2 and a+b = 0 to 2n+2. A process according to claim 3 where the silalkylenes contain one or more silalkylene bonds and are described by formula Si(C) z Si and z = 1, 2 or 3.

Inventors: CHADWICK KIRK MICHAEL, DHAUL AJAY KUMAR, HALM ROLAND LEE, JOHNSON RICHARD GORDON, CHADWICK, KIRK MICHAEL, DHAUL, AJAY KUMAR, HALM, ROLAND LEE, JOHNSON, RICHARD GORDON
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1994
Main classification: C07F7
IPC codes: B01J21, C07F7, C07B61, C01B33, B01J23